The Long-Term Evolution (LTE), whose radio access is called Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), is the latest standard in the mobile network technology tree that previously realized the GSM/EDGE and UMTS/HsxPA network technologies. In light of the progressive deployment of LTE, co-existence with other radio access technologies, such as 3GPP UMTS Terrestrial Radio Access Network (UTRAN) and GSM/Edge Radio Access Network (GERAN), is expected.
In order to give service providers the ability to deliver a seamless mobility experience, it shall allow for end-to-end Quality of service (QoS) during connections and handover to other fixed line and wireless access technologies. LTE is designed to support IP-based voice traffic enabling better integration with other services. However, UTRAN/GERAN provides voice service only via circuit switched (CS) domain. Thus, VCC is an important issue during a handover between the above two radio access networks.
Single Radio Voice Call Continuity (SRVCC) refers to a mechanism designed to maintain voice call continuity between IMS over PS access and CS access for calls while a UE moves from E-UTRAN to UTRAN/GERAN where voice over PS core network is impossible due to network deployment. For facilitating session transfer of the voice component to the CS domain, IMS multimedia telephony sessions need to be anchored in the IMS. FIG. 1 shows a typical handover architecture between E-UTRAN and UTRAN/GERAN. The details ara described in 3GPP TS 23.216, which is incorporated herein by reference in its entirety.
Under some circumstances, however, a voice call is possibly interrupted during a SRVCC handover. FIG. 2 illustrates an example of an interruption which occurs during the SRVCC handover from E-UTRAN to GERAN/UTRAN.
With reference to FIG. 2, at step S201, a UE 20 makes an IMS voice call with a “SRVCC operation possible” indicator set to be “true” in a MME 10, which is a serving node for mobility management for the UE 20 in E-UTRAN. If the UE 20 moves to a new geographical location, then at step S202, a PS handover occurs from the MME 10 to a MME 40, which is another serving node for mobility management for the UE 20 in E-UTRAN, to keep the voice call active, and then at step S203, the MME 40 waits for a TAU Request from the UE 20.
At step S204, it is assumed that an eNodeB 30 initiates a SRVCC handover. Note that at this time, a TAU procedure in the MME 40 has not been triggered. Then at step S205, the SRVCC handover from E-UTRAN to GERAN/UTRAN for the UE 20 is performed. The flow proceeds to step S207, where the UE 20 initiates a RAU procedure by sending a RAU Request message to a SGSN 50, which is a serving node for mobility management for the UE in GERAN/UTRAN. The RAU Request message includes PTMSI derived from GUTI allocated by the MME 10.
At step S208, the SGSN 50 sends a Context Request message in order to obtain information concerning the UE 20 in a Context Response from the MME 10. At this step, because the latest available GUTI used by the UE 20 is allocated by the MME 10, the SGSN 50 sends the request to the MME 10. However, the MME 10 may have no information concerning the UE 20. Specifically, after the PS handover is performed from the MME 10 to a MME 40 at step S202, the MME 10 starts a timer and when the timer expires, it will delete the information at step S206. Therefore, if step S206 occurs prior to step S208, the MME 10 cannot retrieve the information concerning the UE 20 and will return a Context Response message with cause value “imsi unknown”. As a result, at step S209, the SGSN 50 sends a RAU Reject message to the UE 20, which has to initiate an attach procedure again.